Aluminum and its alloys have been used extensively as wiring materials. In other words, they are used to connect components in a semiconductor device. This is because they have many advantages such as low contact resistance, low wiring resistance, good process ability, and availability at reasonable cost, over any other metal.
Various phenomena occur when silicon comes in contact with metal. More specifically, they occur at the contact portion (hereinafter called a contact) between a silicon substrate and a metal wire.
Several of these phenomena are explained, along with the development in LSIs, by making reference to FIGS. 9(a) to 9(d).
As shown in these figures, the scale of integration of semiconductor devices has increased and the device dimensions have decreased.
FIG. 9(a) illustrates one of the early semiconductor devices in which contacts are squares of several micrometers by several micrometers. The semiconductor device of FIG. 9(a) comprises a silicon (Si) substrate 11, a source/drain diffusion region 17, an insulating film 21 which is a planarized film of silicate, and a metal wire 13 which is pure aluminum. The pure aluminum wiring improves controllability because the sputtering process for aluminum wiring involves only aluminum atoms. Also, when an aluminum wiring film deposited is etched by a dry etching process into a desired wiring pattern, good controllability is obtainable, since what is required is only the reaction of aluminum with aluminum chloride.
The above-described pure-aluminum metallization, however, produces a problem, that is, the occurrence of counter diffusion in the aluminum-silicon system. If such counter diffusion takes place, aluminum atoms will enter deep into a silicon diffusion region thereby creating aluminum spikes. Due to the occurrence of aluminum spiking, the metal wire 13 and the silicon substrate 11 may short-circuit. This is one of the causes of defective semiconductors.
Many of the up-to-date semiconductor devices use an organization, shown in FIG. 9(b), in order to eliminate aluminum spikes. FIG. 9(b) shows two different contact organizations. One, on the left side of the figure, is a metal wire 13B which is aluminum that contains several percent of silicon. The other, on the right side, is characterized in that a barrier metal 15a is sandwiched between a metal wire 13A which is pure aluminum and the silicon substrate 11.
In the case of the silicon-containing aluminum wiring type, as in the metal wire 13B, the solid solubility of silicon in aluminum is below 1% at temperatures not exceeding several hundred degrees centrigrade. No diffusion from the silicon substrate 11 into the aluminum occurs, and the occurrence of spiking is prevented. In the case of the barrier metal type, a film of titanium nitride (TIN) having a film thickness of about 100 nanometers is used to serve as the barrier metal 15a. In high-melting-point metal type materials such as TiN, their inter-lattice gap is so small that no aluminum atoms can make their way through it. Therefore, the occurrence of counter diffusion in the Al-Si system is restrained whereby the occurrence of spiking is prevented.
The silicon-containing aluminum wiring type, however, has a problem that an excess of silicon is expelled from the metal wire 13B. Such expelled silicon is likely to build up at the interface between the metal wire 13B and the semiconductor substrate 11. Particularly, for the case of approximately 1 .mu.m.times.1 .mu.m contacts, they will be entirely covered with the expelled silicon. In the case of n.sup.+ -type diffusion regions, the silicon, expelled and built up at a contact, slightly contains aluminum so that it exhibits a p-type characteristic. Generally, ohmic contact is formed between a diffusion region and a metal wire. If a p-type silicon is inserted between a diffusion region of n.sup.+ -type and the metal wire 13B, however, this breaks the ohmic contact formation. The contact resistance between the n.sup.+ -type diffusion region and the metal wire 13B becomes greater, and thus the characteristic of transistors degrades dramatically.
In the case of the barrier metal type organization, on the left side of FIG. 9(b), it is necessary to carry out an extra sputtering process to deposit the barrier metal 15a before the deposition of aluminum by a later sputtering process. This requires extra equipment.
There is a common problem to both of these two types. As the dimensions of the contacts decrease, the step coverage of the aluminum films formed by a sputtering process at the contact dramatically drops. An example of such a drop in step coverage is shown in FIG. 9(c). In this example, a 1 .mu.m-thick aluminum film is deposited by a sputtering process on a semiconductor device having contact holes (hole size: 1 .mu.m.times.1 .mu.m, depth: 1 .mu.m). The aspect ratio (i.e., depth/width of holes) is 1:1 here. The deposited aluminum has a very thin film thickness at the bottom of the contact hole, below one-tenth of the film thickness at the other locations (i.e., below 0.1 .mu.m thick). If electric current flows through such a thin wiring portion, disconnection, due to heat developed by the increase in resistance, may take place.
An improved semiconductor structure of FIG. 9(d) has been proposed with the aim of providing a solution to the above-described problem of FIG. 9(c). FIG. 9(d) shows two different contact formation techniques, both of which employs a CVD (chemical vapor deposition) process that utilizes a gas of WF.sub.6. In a CVD process, films can be deposited with good step coverage even at a step as well as at the bottom of a contact hole, since the chemical reaction of the source gas mainly occurs on the surface of a semiconductor substrate. The barrier metal 15a is a film of TiN having a film thickness of about 100 nanometers. An aluminum metal wire is indicated by reference numeral 13'.
In one of the two contact formation techniques shown in FIG. 9(d), a tungsten (W) film 14 is selectively deposited by a selective CVD process so that the tungsten film 14 is deposited only on top of a contact portion. In the other of the two contact formation techniques, on the other hand, a film of TiN is formed all over the silicon substrate 11 and the insulating film 21. This is followed by the deposition of a tungsten film by means of a blanket CVD process. Then, the deposited W and TiN films are etched to obtain electrical contact. The barrier metal 15a, comprised of the tungsten film 14 and the TiN film, is left in a contact hole.
These two contact formation techniques can prevent metal films from becoming thin, since the contact hole is filled with the tungsten film 14.
Both the selective CVD process and the blanket CVD process make use of a gas of WF.sub.6 as a source gas. Tungsten is deposited as follows. EQU WF.sub.6 +3H.sub.2 .fwdarw.W+6HF (1) EQU 5WF.sub.6 +6SiH.sub.4 .fwdarw.5W+24HF+6SiF (2)
As seen from the chemical formula (2), WF.sub.6 easily reacts with silicon. Because of such a chemical reaction, the TiN film becomes thin during the selective or the blanket CVD process and the following chemical reaction will occur if the silicon in an underlying film is exposed. EQU WF.sub.6 +6Si.fwdarw.W+6SiF (3)
This replaces the silicon with the tungsten thereby creating tungsten encroachments and wormholes on the diffusion region 17. As a result, the same problems as accompanied with the occurrence of spiking arise.
As described above, use of aluminum in metallization results in the formation of spikes. Additionally, when filling a contact hole with a tungsten film, the tungsten reacts with the silicon in the diffusion region. As a result, the tungsten enters into the diffusion region causing short-circuiting between the silicon substrate and an electrode.
These problems occur not only when forming a metal film, of aluminum or tungsten, on a semiconductor substrate but also when forming a metal film on metal wiring formed on or over a semiconductor substrate.